Safety lock for bascule bridge

ABSTRACT

A bascule bridge is pivoted between a horizontal position and an erect position by hydraulic cylinders. At each side of the bridge adjacent the pivotal end is a wing. These wings are moved by hydraulic cylinders between a position at which they are about in the same plane as the sides of the bridge, in which position the bridge may be raised and lowered, and, when the bridge is in the erect position the wings are automatically moved outwardly to form supports to prevent the bridge from accidentally lowering.

United States Patent [191 Richards et al.

[451 Nov. 19, 1974 1 SAFETY LOCK FOR BASCULE BRIDGE [75] Inventors: Lawrence I. Richards; Ronald R.

Gould, both of Elk Grove Village,

[73] Assignee: Autoquip Corporation, Chicago, 111.

[22] Filed: May 14, 1973 [21] Appl. No.: 360,118

[52] US. Cl. 14/41 [51] Int. Cl E0ld 15/06 [58] Field of Search 14/41, 39, 38, 61, 69; 16/167, 191; 292/338, 262; 108/77; 312/244; 248/2404 [56] References Cited UNITED STATES PATENTS 193,046 7/1877 Smith 248/240.4 379,861 3/1888 Cornell 14/71 X 1,042,238 10/1912 Krase 14/41 3,192,546 7/1965 Noland 14/72 3,255,478 6/1966 Lambert 14/71 3,290,710 12/1966 Whitenack... 14/71 3,332,373 7/1967 Ruiz 108/77 3,456,274 7/1969 McGuire 14/71 3,516,103 6/1970 Hecker 14/71 Primary Examiner-Nile C. Byers Jr. Attorney, Agent, or Firm-Darbo, Robertson & Vandenburgh [5 7] ABSTRACT 10 Claims, 5 Drawing Figures 1 SAFETY LOCK FOR BASCULE BRIDGE BACKGROUND AND SUMMARY OF THE INVENTION Bascule bridges are commonly used in manufacturing plants and the like having facilities on both sides of a roadway or a railroad siding. The bridge can be raised to an erect position to permit traffic along the roadway or siding. When there is no necessity for such traffic, the bridge is lowered to a horizontal position to permit traffic across the bridge from the facility at one side to the facility at the other side and vice versa.

It is desirable, and indeed even mandatory in some instances, that a safety device be provided to prevent the bridge from accidentally lowering from the erect position. For example, such a bridge may be outdoors and thus subject to wind pressure which can act to force the bridge in the lowering direction; or there may be a failure in the apparatus used to move the bridge to the erect position and retain it there. The common form of such safety devices is a latch which is positioned between some stationary framework and the distal end of the bridge when the bridge is in the erect position. Such latches can functionally serve the purpose, but in many instances they involve a substantial additional expense to install. For example, when the distal end of the bascule bridge in the raised position is not immediately adjacent a preexisting fixed framework (e. g. the side of a building) it becomes rather expensive to install a suitable stationary framework at the required location.

The principal object of the present invention is to provide an alternative safety device for a bascule bridge to maintain it in the erect position, which safety device will be relatively inexpensive and be fully adequate to do the job. A further advantage of the present invention is that this safety device can be fabricated as a part of the bascule bridge while the bridge is being made in a factory and substantially little additional field construction is required over that necessary for the prior art bridge apparatus. Further objects and advantages will become apparent from the following description taken in conjunction with the drawings.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of the proximal end of a bascule bridge embodying the present invention;

FIG. 2 is a longitudinal section taken at the proximal end of the bridge, with the bridge in the erect position;

FIG. 3 is a horizontal section taken at line 33 of FIG. 2;

FIG. 4 is a schematic electrical diagram of the control apparatus; and

FIG. 5 is a schematic hydraulic diagram of the control apparatus.

DESCRIPTION OF SPECIFIC EMBODIMENT The following disclosure is offered for public dissemination in return for the grant of a patent. Although it is detailed to ensure adequacy and aid understanding, this is not intended to prejudice that purpose of a patent which is to cover each new inventive concept therein no matter how others may later disguise it by variations in form or additions or further improvements.

In the drawings there is a bascule bridge, generally 10. It comprises a suitable frame 10a fonned of longitudinal channels 11 and a series of cross channels 12. Over this framework there is a deck 13. The proximal end of the bridge is supported on a suitable foundation 14. The proximal end of the bridge frame is connected by a shaft 15 to a suitable part (e.g. the imbedded channel) of the foundation 14 (forming a pivotal bridge support) so that the bridge can be pivoted from the horizontal position (FIG. 1) to an erect position (FIG. 2). The pivotal movement is accomplished by a suitable power device, commonly consisting of one or more hydraulic cylinders, generally 16 (two being used in the illustrated structure). One end of the cylinders is pivotally connected to the frame 10a of bridge 10 and the other end is pivotally connected to a suitable mounting forming a part of foundation 14. As thus far described, the structure is conventional.

The safety devices of the present invention comprise a pair of wing members 19, one being used at each side of the bridge immediately adjacent the proximal end thereof. These wing members are pivotally connected by hinges 20 to the bridge frame. When the bridge is in the erect position the wing members are pivotable between (I) an outwardly extending position (full lines in FIG. 3) at which they serve as blocking means to prevent the descent of the bridge from the erect position, and (2) a retracted position (shown in phantom in FIG. 3) at which they no longer serve the blocking function and will permit the descent of the bridge. The wings have a pad 21 across the end thereof. When the bridge is in the erect position and the wings extended. this pad 21 is just above the top face areas 22 of foundation 14, which areas serve as abutments which, upon contact between pads 21 and areas 22, will immovably resist any further downward movement of pads 21, and thus of the bridge 10.

The movement of the wing members 19 is achieved by double-aacting hydraulic cylinders 24. Each wing member 19 has a limit switch 25 to detect when the wing member is fully retracted and a limit switch 26 to detect when the wing member is fully extended. These switches are mounted on the bridge frame with the switch actuator being adjustably connected to the wing member. In FIG. 4, the limit switches 25' and 26' are associated with one wing member and the switches 25" and 26" are associated with the other wing member. There is a bridge position limit switch 27 to determine whether the bridge is in the erect position. In the illustrated embodiment this position switch 27 is mounted on the bridge with its actuator positioned to be moved by contact with one of the hydraulic cylinders 16 when the bridge is in the erect position. The foundation 14 forms a cavity 28 within which the proximal end of the bridge is recieved. This cavity also provides space for the hydraulic and electrical power unit 29.

l The hydraulic system is best seen by reference to FIG. 5. There is a hydraulic pump 31 driven by an electrical motor 32. The pump draws hydraulic fluid from a sump or reservoir 33 through an intake pipe 34. There is a filter 35 in this intake pipe.

The pump discharges the hydraulic fluid under pressure into a supply pipe 36. From this supply pipe there is a return pipe 37 having a relief valve 38 therein. This prevents the fluid pressure in supply pipe 35 from exceeding a predetermined requisite amount. From supply pipe 36 the hydraulic fluid flows through a check valve 41 to a second supply pipe 42. Supply pipe 42 has branches leading to hydraulic cylinders 16, a branch leading to a pressure compensated flow control valve 43 and to a valve, generally 44. From flowcontrol valve 43 a pipe 45 having a down solenoid valve, generally 46, therein returns to reservoir 33. From valve 44 there is a pipe 47 leading to one of the ends of the hydraulic cylinders 24, a pipe 48 leading from the other of the ends of the hydraulic cylinders and a pipe 49 communicating with reservoir 33.

Valve 46 has two positions, namely open and closed. It is normally maintained in the closed position by a spring 51. When a solenoid 52 forming a part of the valve is energized, the valve is opened to permit hydraulic fluid from pipe 42 to return to the reservoir 33. The valve 44 is a three-position valve. It is normally maintained in the centered position by springs 53 and 54. In this centered position all of the pipes connecting with the valve are shut off from each other. When a solenoid 56 is energized, the resistance of spring 54 is overcome so that the valve is changed to a second position at which pipes 42 and 47 are in communication and pipes 48 and 49 are in communication. This second position is used to extend the hydraulic cylinders 24 and thus extend the wings 19. When the solenoid 55 is energized the valve is changed to a third position (against the resistance of spring 53) at which the pipes 47 and 49 are in communication with each other and the pipes 42 and 48 are in communication with each other. This third position acts to retract the hydraulic cylinders and thus the wings 19.

The electrical circuit is illustrated in FIG. 4. The electrical motor 32 is a three-phase (230 or 460 volt) motor. It is connected to the line (source of electrical power) through a main switch and fuse box 60, the main contacts (switches) 61 of a magnetic starter and thermal overloads 62. The magnetic starter includes an actuating solenoid 63 and an auxiliary switch 64.

The accessible electrical controls are connected to the secondary of an isolation transformer 66. The primary of this transformer is connected to one phase ofthe main power supply (ahead of the starter switches 61). The electrical controls includes two up pushbutton switches 67 and 68 each comprising a battery of four ganged switches 67a 67d and 68a 68d respectively. These switches are normally held in the illustrated position by springs 67e and 68e. Similarly, there are a pair of down-switches 69 and 70. These are push-button operated against the resistance of springs 69e and 702. Each includes individual switches 69a 69d and 70a 70d respectively. From the standpoint of physical arrangement, there would be one of the upswitches and one of the down-switches (e.g. 67 and 69) positioned so as to be accessible from one end of the bascule bridge and a second pair of the up and down switches (eg 68 and 70) positioned so as to be accessible from beyond the other end of the bascule bridge. The electrical power from the secondary of transformer 66 is supplied to two wires 72 and 73, there being a fuse 74 in one of these wires. From wire 72 the switches 67a and 68a (normally open) are connected in parallel to a wire 75. From wire 75 the normally closed switches 69a and 70a are connected in series to a wire 76. From wire 76 the limit switches 26' and 26" are connected in parallel to a wire 77. Wire 77 connects to the starter solenoid 63 and to the alternate contacts of limit switches 25 and 25 The other side of solenoid 63 is connected through an overload switch 78 to wire 73.

Switches 67b and 68b are connected in parallel from wire 72 to a wire 79. From wire 79 switches 69b and b are connected in series to a wire 80. Wire 80 connects to bridge position switch 27. From the other side of this bridge position switch, one contact is connected by a wire 81 to solenoid 56 of valve 44 and the other contact of the position switch 27 is connected by a wire 82 to solenoid 55 of the same valve. The other ends of solenoids 55 and 56 connect to power supply wire 73. Wire 82 also connects to one side of auxiliary switch 64 of the magnetic starter.

Normally closed switches 67c and 680 are connected in series from wire 72 to a wire 83. From wire 83 normally open switches 69c and 70c are connected in parallel to a wire 84. Wire 84 connects to limit switch 25'. A wire 85 connects one side of limit switch 25 to limit switch 25". A wire 86 connects one side of limit switch 25" to solenoid 52 of valve 46. The other end of the solenoid is connected to wire 73.

Normally closed switches 67d and 68d are connected in series from wire 72 to a wire 87. Normally open switches 69d and 70d are connected in parallel between wire 87 and wire 88. Wire 88 connects from this pair of parallel switches to auxiliary switch 64.

OPERATION It is assumed that main switch 60 is closed and the system is otherwise ready to operate. The various switch positions illustrated in FIG. 4 are with the bridge lowered (and thus the wing members 19 retracted) and with no effort being made to raise or lower. When the bridge is to be raised one or the other of the upswitches 67 or 68 is depressed so as to change the position of the switch contacts thereof. The thus closed upper switch contact 67a or 68a energizes wire 75. Through switches 69a, 70a and one or the other of 26' or 26", the solenoid 63 is thereby energized. This closes switches 61 and 64. The closing of switch 64 has no significance at this time. The closing of switches 61 energizes motor 32 to commence operating pump 31 to supply hydraulic fluid to the lifting cylinders 16. This, of course, commences the raising of the bridge.

At the same time, the closing of switch 67b or 68b energizes wire 79. In turn, this energizes solenoid 55 which is the wing member 19 retract solenoid. That is, it positions valve 44 so that hydraulic fluid is supplied to cylinders 24 in the direction such as to retract the wing members 19. This maintains them in the already retracted positions.

It will be noted that both of the electrical circuits just discussed extend through (are in series with) normally closed switches 69a, 70a, 69b and 70b of downswitches 69 and 70. Thus, should the down-button be pressed while the up-button is also depressed, these two electrical circuits are broken.

When the bridge reaches the erect position, the bridge position switch 27 is actuated so that the arm thereof moves to the upper (as viewed in FIG. 4) contact. This now energizes solenoid 56 (and deenergizes solenoid 55) to reverse the position of the valve 44. This causes the hydraulic fluid to be supplied to cylinders 24 in the opposite sense, namely, in a manner so as to cause the cylinders to extend and thus swing the wings 19 outwardly. When the wings have reached the full extent of their outward position (i.e. have moved to the full line position in FIG. 3) the limit switches 26 and 26" are respectively actuated. When both of them have been actuated this opens the circuit through solenoid 63 of the starter causing the switches 61 and 64 to return to the illustrated position. This stops motor 32. The full operation having been achieved, the operator can now release the up-switch 67 or 68 which he had previously been holding in the depressed position. Should he have released the depressed up-switch prematurely, all action would have stopped since the above discussed electrical circuits would have been thereby broken.

Now, assuming that the bridge is to be lowered, one of switches 69 or 70 is manually actuated (i.e. depressed against the resistance of the respective spring 692 or 70e). Through closed switches 67c and 68c an electrical circuit is created from wire 72 through the closed down-switch 690 or 70c to wire 84. Previously, when the wings commenced moving away from the retracted position, the switches and 25" moved to their alternate position. Thus, through these limit switches, electrical power is supplied from wire 84 to wire 77. This energizes solenoid 63 to close switches 61 and 64. The closing of switches 61 energizes motor 32 to commence the operation of pump 31, as previously discussed. The switch 27 is in the upper position of FIG. 4, but does not energize solenoid 56 because one of switches 69b or 70b are open. The closing of switch 64 completes an electrical circuit from wire 72 through switch 67d, switch 68d, one or the other of switches 69d and 70d to thereby energize solenoid 55.

The second fluid supply pipe 42 will be pressurized because the pump has started. This energizes hydraulic cylinder 16 in the direction so as to raise the bascule bridge. Thus, if the bridge had settled so that pads 21 were in contact with abutments 22 this contact would be removed by raising the bridge to the FIG. 2 position. At the same time, the energizing of solenoid 55 sets valve 44 so as to power the hydraulic cylinders 24 in the retraction direction. This would cause the wings 19 to commence moving in. Initially during that movement, the limit switches 26' and 26" close, but this has no effect since either switch 69a or switch 70a is open.

When the wings are fully retracted, switches 25' and 25" are moved to the position illustrated in FIG. 4. This deenergizes solenoid 63 so as to open switches 61 and 64 to respectively turn off motor 32 and deenergize solenoid 55. With neither of solenoids 55 or 56 being energized, the valve 44 returns to the position illustrated in FIG. 5. However, the down push buttom 69 or 70 (whichever is being depressed) continues to supply electric power to wire 84 and thus through limit switches 25 and 25" to solenoid 52. This energizing of solenoid 52 opens valve 46 to permit hydraulic fluid to return from the second supply pipe 42 to the reservoir 33. Thus, hydraulic cylinders 16 are permitted to retract and, under the weight of the bridge 10, the bridge descends. The rate of descent is controlled by valve 43 which restricts the flow of hydraulic fluid through the return pipe 45.

As the bridge commences leaving the erect position,

the switch 27 returns to the position illustrated in FIG.

4, but this has no effect at this time. When the bridge is in the fully lowered (horizontal) position, the operator releases the depressed down-switch button. This deenergizes solenoid 52 permitting valve 46 to return to the closed position. The apparatus now is ready to repeat the operation commencing with the beginning of the foregoing description. It will be noted that if an upbutton had been pressed at the time that the down button had been depressed, the electrical circuits leading to wires 84 and 88 would have been broken by the opening of switches 67c, 67d or 68c, 68d.

If the operator depresses a down (or up) button, thus commencing the described cycle, and then releases that button in the middle of the cycle, the operation will stop at that point. Thereafter, the operator can again press the same button and the cycle will recommence from the point of stoppage. If the button of the opposite function is pressed instead, the opposite cycle will commence from that point.

We claim:

1. In a bascule bridge apparatus for spanning an area which apparatus comprises a foundation member at one side of said area and forming a pivotal bridge support, a bridge member having one end pivotally mounted on said support, a power device connected to the bridge member to pivot the bridge member between a horizontal position and an erect position, and a safety device to hold the bridge member in the erect position to prevent accidental lowering of the bridge member, the improvement wherein:

said foundation member forming abutment means immediately adjacent said support;

said safety device including blocking means movably attached to one of said members adjacent said one end of the bridge member and movable, only when said bridge member is in the erect position,

to a first position in juxtaposition to the other member so as to contact said other member should said bridge member start to lower and thereby block further downward movement of the bridge member, and

to a second position at which the blocking means is clear of said other member so that the bridge member can freely move down and up, and

power means interconnecting the one member and the blocking means to move said blocking means between its two positions.

2. In a bascule bridge apparatus for spanning an area which apparatus comprises a foundation at one side of said area and forming a pivotal bridge support, a bridge having one end pivotally mounted on said support, a power device connected to the bridge to pivot the bridge between a horizontal position and an erect position, and a safety device to hold the bridge in the erect position to prevent accidental lowering of the bridge, the improvement wherein:

said foundation forming abutment means immediately adjacent said support;

said safety device including blocking means movably attached to said bridge adjacent said one end and movable, only when said bridge is in the erect position,

to a first position in juxtaposition to said abutment means so as to contact said abutment means should said bridge start to lower and thereby block further downward movement of the bridge, and

to a second position at which the blocking means is clear of said abutment means so that the bridge can freely move down and up, and

power means interconnecting the bridge and the blocking means to move said blocking means between its two positions.

3. In an apparatus as set forth in claim 2, wherein said power means includes a control switch actuated when said bridge about reaches said erect position to render said blocking means movable between its two positions only when the bridge is about in its erect position.

4. In an apparatuus as set forth in claim 3, wherein said blocking means includes two wing members, each member being at a respective side of the bridge and pivotally connected thereto.

5. In an apparataus as set forth in claim 4, wherein said abutment means includes an abutment at each side of said bridge extending from the proximal end of the bridge toward the distal end, the pivotal connections of the wing members being along the sides of the bridge adjacent the proximal end of the bridge, said wing members in said first position extending outwardly frorn the sides of the bridge and in said second position being approximately in the 35121366? said sides, s aid wing members having ends overlying said abutments when in the first position. I

6. In an apparatus as set forth in claim 5, wherein said power means includes a source of hydraulic fluid under pressure, a pair of hydraulic cylinders with each being connected to a respective wing member, fully extended limit switches with one being for each member respectively, fully retracted limit switches with one being for each member respectively, and circuit means connected to said power device and to said limit switches for raising and lowering said bridge only when said members are retracted to said second position.

7. In an apparatus as set forth in claim 6, wherein said cylinders are double acting, said power means includes solenoid operated valve means for controlling the flow of hydraulic fluid from said source to said cylinders, and wherein said circuit means includes a bridge position switch, an up-switch means, a down-switch means, said up-switch means and said down-switch means being connected in series so that when one is actuated the other is deactivated and vice versa, said up-switch means being connected to said valve means, said bridge position switch, and said limit switches to actuate the cylinders to move said members to the first position when the up-switch means is operated and the bridge reaches said erect position, said down-switch means being connected to the valve means, said bridge position switch, said power device and the limit switches to retract the members to the second position from the first position and to thereafter lower said bridge to the horizontal position.

8. In an apparatus as set forth in claim 2, wherein said power means includes a source of hydraulic fluid under pressure, a hydraulic cylinder connected to the blocking means, a fully extended limit switch for said blocking means, a fully retracted limit switch for said blocking means, and circuit means connected to said power device and to said limit switches for raising and lowering said bridge only when said blocking means is retracted to said second position.

9. In an apparatus as set forth in claim 8, wherein said cylinder is double acting, said power means includes solenoid operated valve means for controlling the flow of hydraulic fluid from said source to said cylinder, and wherein said circuit means includes a bridge position switch, an up-switch means, a down-switch means, said up-switch means and said down-switch means being connected in series so that when one is actuated the other is deactivated and vice versa, said up-switch means being connected to said valve means, said bridge position switch, and said limit switches to actuate the cylinder to move said blocking means to the first position when the up-switch means is operated and the bridge reaches said erect position, said down-switch means being connected to the valve means, said bridge position switch, said power device and the limit switches to retract the blocking means to the second position from the first position and to thereafter lower said bridge to the horizontal position.

10. In an apparatus as set forth in claim 9, wherein when the bridge is erect and the blocking means in the first position, the down-switch means when operated actuates the power device to apply a raising force to the bridge until said blocking means has moved to the second position and only thereafter to lower the bridge. 

1. In a bascule bridge apparatus for spanning an area which apparatus comprises a foundation member at one side of said area and forming a pivotal bridge support, a bridge member having one end pivotally mounted on said support, a power device connected to the bridge member to pivot the bridge member between a horizontal position and an erecT position, and a safety device to hold the bridge member in the erect position to prevent accidental lowering of the bridge member, the improvement wherein: said foundation member forming abutment means immediately adjacent said support; said safety device including blocking means movably attached to one of said members adjacent said one end of the bridge member and movable, only when said bridge member is in the erect position, to a first position in juxtaposition to the other member so as to contact said other member should said bridge member start to lower and thereby block further downward movement of the bridge member, and to a second position at which the blocking means is clear of said other member so that the bridge member can freely move down and up, and power means interconnecting the one member and the blocking means to move said blocking means between its two positions.
 2. In a bascule bridge apparatus for spanning an area which apparatus comprises a foundation at one side of said area and forming a pivotal bridge support, a bridge having one end pivotally mounted on said support, a power device connected to the bridge to pivot the bridge between a horizontal position and an erect position, and a safety device to hold the bridge in the erect position to prevent accidental lowering of the bridge, the improvement wherein: said foundation forming abutment means immediately adjacent said support; said safety device including blocking means movably attached to said bridge adjacent said one end and movable, only when said bridge is in the erect position, to a first position in juxtaposition to said abutment means so as to contact said abutment means should said bridge start to lower and thereby block further downward movement of the bridge, and to a second position at which the blocking means is clear of said abutment means so that the bridge can freely move down and up, and power means interconnecting the bridge and the blocking means to move said blocking means between its two positions.
 3. In an apparatus as set forth in claim 2, wherein said power means includes a control switch actuated when said bridge about reaches said erect position to render said blocking means movable between its two positions only when the bridge is about in its erect position.
 4. In an apparatuus as set forth in claim 3, wherein said blocking means includes two wing members, each member being at a respective side of the bridge and pivotally connected thereto.
 5. In an apparataus as set forth in claim 4, wherein said abutment means includes an abutment at each side of said bridge extending from the proximal end of the bridge toward the distal end, the pivotal connections of the wing members being along the sides of the bridge adjacent the proximal end of the bridge, said wing members in said first position extending outwardly from the sides of the bridge and in said second possition being approximately in the plane of said sides, said wing members having ends overlying said abutments when in the first position.
 6. In an apparatus as set forth in claim 5, wherein said power means includes a source of hydraulic fluid under pressure, a pair of hydraulic cylinders with each being connected to a respective wing member, fully extended limit switches with one being for each member respectively, fully retracted limit switches with one being for each member respectively, and circuit means connected to said power device and to said limit switches for raising and lowering said bridge only when said members are retracted to said second position.
 7. In an apparatus as set forth in claim 6, wherein said cylinders are double acting, said power means includes solenoid operated valve means for controlling the flow of hydraulic fluid from said source to said cylinders, and wherein said circuit means includes a bridge position switch, an up-switch means, a down-switch means, said up-switch means and said down-switch means being connected in series so that when one is actuated the other is deactivated and vice versa, said up-switch means being connected to said valve means, said bridge position switch, and said limit switches to actuate the cylinders to move said members to the first position when the up-switch means is operated and the bridge reaches said erect position, said down-switch means being connected to the valve means, said bridge position switch, said power device and the limit switches to retract the members to the second position from the first position and to thereafter lower said bridge to the horizontal position.
 8. In an apparatus as set forth in claim 2, wherein said power means includes a source of hydraulic fluid under pressure, a hydraulic cylinder connected to the blocking means, a fully extended limit switch for said blocking means, a fully retracted limit switch for said blocking means, and circuit means connected to said power device and to said limit switches for raising and lowering said bridge only when said blocking means is retracted to said second position.
 9. In an apparatus as set forth in claim 8, wherein said cylinder is double acting, said power means includes solenoid operated valve means for controlling the flow of hydraulic fluid from said source to said cylinder, and wherein said circuit means includes a bridge position switch, an up-switch means, a down-switch means, said up-switch means and said down-switch means being connected in series so that when one is actuated the other is deactivated and vice versa, said up-switch means being connected to said valve means, said bridge position switch, and said limit switches to actuate the cylinder to move said blocking means to the first position when the up-switch means is operated and the bridge reaches said erect position, said down-switch means being connected to the valve means, said bridge position switch, said power device and the limit switches to retract the blocking means to the second position from the first position and to thereafter lower said bridge to the horizontal position.
 10. In an apparatus as set forth in claim 9, wherein when the bridge is erect and the blocking means in the first position, the down-switch means when operated actuates the power device to apply a raising force to the bridge until said blocking means has moved to the second position and only thereafter to lower the bridge. 